A solar cell converting sunlight to electric power has attracted attention as a source of energy replacing fossil fuel. A solar cell including a crystalline silicon substrate and a thin-film silicon solar cell have currently been put into practical use. The former, however, is disadvantageous in high cost for fabricating a silicon substrate, and the latter is disadvantageous in high manufacturing cost because of necessity for use of various semiconductor manufacturing gases or a complicated apparatus.
Therefore, though efforts for reducing cost per generated power output have been continued by enhancing efficiency in photoelectric conversion in any solar cells, the problems above have not yet been solved.
Japanese Patent Laying-Open No. 1-220380 (PTD 1) is a prior document disclosing a dye-sensitized solar cell achieved by applying photo-induced electron transfer of metal complex as a solar cell of a new type. The dye-sensitized solar cell described in PTD 1 has such a structure that a photoelectric conversion layer having an absorption spectrum in a visible light region by adsorbing a photosensitizing dye and an electrolytic solution are sandwiched between a glass substrate having a first electrode formed on a surface thereof and a glass substrate having a second electrode formed on a surface thereof.
In the dye-sensitized solar cell, as a transparent first electrode side is irradiated with light, electrons are generated in the photoelectric conversion layer. Generated electrons are transferred from the first electrode through an external electric circuit to the opposed second electrode. Transferred electrons are carried by ions in an electrolyte, and they return to the photoelectric conversion layer. Electric energy can be taken out through repetition of such a series of electron transfer.
The dye-sensitized solar cell described in PTD 1, however, has such a basic structure that an electrolytic solution is introduced in between two glass substrates. Therefore, though it is possible to prototype a solar cell having a small area, it is difficult to apply this structure to a solar cell having an area as large as 1-m square. This is because, as an area of a single solar cell is increased, a short-circuit current increases in proportion to the area, however, resistance in an in-plane direction of the first electrode increases, which leads to increase in internal series electrical resistance of the solar cell and consequently, lowering in fill factor (FF) in current-voltage characteristics at the time of photoelectric conversion and resultant failure in obtaining sufficient efficiency in photoelectric conversion.
In order to solve the problems above, Japanese Patent Laying-Open No. 2003-203681 (PTD 2) as a prior art document discloses a photoelectric conversion element achieving lowering in electrical resistance by forming a collector electrode on a first electrode. In the photoelectric conversion element described in PTD 2, a lattice-shaped collector electrode made of an alloy of gold and silver is formed on the first electrode between a plurality of strip-shaped photoelectric conversion portions formed on the same plane. This collector electrode can improve FF and short-circuit current density.